Method for producing a semiconductor component and a semiconductor component produced according to this method

ABSTRACT

A method for manufacturing a semiconductor component ( 100; . . . ; 700 ), a multilayer semiconductor component in particular, preferably a micromechanical component, such as a heat transfer sensor in particular having a semiconductor substrate ( 101 ), in particular made of silicon, and a sensor region ( 404 ).  
     For inexpensive manufacture of a thermal insulation between the semiconductor substrate ( 101 ) and the sensor region ( 404 ) a porous layer ( 104; 501 ) is provided in the semiconductor component ( 100; . . . ; 700 ).

BACKGROUND INFORMATION

[0001] The present invention is directed to a method for manufacturing a semiconductor component, such as in particular a multilayer semiconductor component, and a semiconductor component manufactured by this method according to the definition of the species of the corresponding independent Claim.

[0002] Semiconductor components, in particular micromechanical heat transfer sensors, are usually manufactured by bulk or surface micromechanical methods. Manufacturing bulk micromechanical components is relatively complex and therefore expensive. With the known surface micromechanical components, it is complicated to manufacture a cavern. A conventional process sequence for manufacturing a cavern by surface micromechanics involves in particular deposition of a sacrificial layer, deposition of a membrane layer, usually polysilicon, creating openings in the membrane layer and/or opening a lateral etching channel, etching out the sacrificial layer and sealing the openings, with the inside pressure of the cavern being defined during sealing.

ADVANTAGES OF THE INVENTION

[0003] The method according to the present invention having the characterizing features of the corresponding independent Claim has in particular the advantage over the related art that a micromechanical component, such as a heat transfer sensor in particular having sensor elements, may be manufactured easily and inexpensively by surface micromechanics. Due to the measures cited in the dependent claims, advantageous refinements of and improvements upon the method and the semiconductor component according to the corresponding independent claims are made possible.

[0004] A heat transfer sensor is to be understood in particular as being a sensor for measuring an air stream and/or a gas stream, an air pressure and/or a gas pressure on the basis of the heat transferred to the air and/or the gas, such as hydrogen in particular.

[0005] An important aspect of the present invention is to create a porous layer in a semiconductor component, e.g., a heat transfer sensor. The porous layer may be formed in a semiconductor substrate, such as in a silicon substrate in particular or in an epitaxial layer, e.g., a silicon layer, deposited on the semiconductor substrate. The porous inexpensively manufacturable layer in particular serves the function of thermal insulation of the semiconductor substrate and/or the epitaxial layer with respect to a sensor area provided in the semiconductor component. Because of the thermal insulation, a heat flow from the sensor area to the substrate and/or the epitaxial layer is greatly reduced. The response sensitivity of a sensor provided in the sensor area is increased and its response time is shortened. To do so, the cover layer of the substrate or the epitaxial layer is etched in the area of the sensor elements to be manufactured later, so that openings, i.e., etching openings, such as pores or cavities in particular are formed in these sensor elements. The thickness and/or porosity of the porous layer is adjustable or variable as needed by the etching process which is explained below. For example, the thickness of the porous layer is in the range of approximately 2 μm to 80 μm.

[0006] Another important aspect of the present invention is to create a cavern, i.e., a cavity in the semiconductor substrate or in the epitaxial layer using an etching medium in the area of the porous layer or underneath it in order to further improve upon the thermal insulation, i.e., isolation of the substrate and/or the epitaxial layer with respect to the sensor area. The etching medium or one or more additional etching media reach deeper regions of the substrate or the epitaxial layer through the etching openings, i.e., the pores of the porous layer which are open to the outside. The part of the semiconductor substrate, i.e., the epitaxial layer, which is decomposed by the etching medium and/or additional etching media in this area is preferably removed through the openings, i.e., pores in the cover layer and/or through an external access opening to this area. The porous layer or cover layer above the cavity or cavern is preferably thin and at the same time is designed with adequate load bearing capacity so that additional layers may be deposited on the porous layer and structured. The porous layer above the cavity preferably has a thickness of approximately 2 μm to 10 μm, in particular 3 μm to 5 μm.

[0007] In a preferred embodiment of the present invention, measures are taken in the etching process to ensure that the rate of expansion of the pores in the cover layer is lower, preferably much lower, than the rate of expansion of the pores or cavities in the area of the substrate or the epitaxial layer which forms the subsequent cavity or cavern.

[0008] This is achieved according to an advantageous embodiment of the present invention by selecting-the etching parameters and/or the etching medium or etching media in etching the pores in the cover layer that are different from the etching parameters and/or the etching medium or etching media in etching the pores or cavities in the area of the subsequent cavern.

[0009] It is particularly advantageous here that for removal of the silicon to be decomposed to manufacture the cavern, the porosity of the cover layer is preferably adjustable to be merely appropriately high in a manner that is easily controllable in terms of the process technology. However, the cavern may be manufactured rapidly and thus inexpensively.

[0010] According to a preferred embodiment of the present invention, the etching parameters may be adjusted and/or the etching media in etching the cavern may be selected so that the rate of expansion of the pores or cavities beneath the porous cover layer is so high that the pores or cavities “overlap” one another very rapidly. At first this results in a single initially superficial starting cavity in the substrate or in the epitaxial layer which expands in depth over time and forms the cavern.

[0011] In a preferred alternative embodiment of the present invention, which is an alternative to the previous embodiment, the etching parameters and/or the etching medium or media should be selected in etching the cavern so that the porosity of the region of the substrate or the epitaxial layer which will later form the cavern is higher than the porosity of the cover layer. The precursor of the subsequent cavern preferably has a porosity higher than 70% with respect to the ratio of the volume of all the cavities of the cavern to the volume of the material of the cavern that has not been etched out. The cavern is preferably formed subsequently from the porous region of the substrate or the epitaxial layer in executing one or more heating steps, preferably to a temperature above approximately 900° C.

[0012] During heating, preferably in an atmosphere of hydrogen, nitrogen, or noble gas as at a temperature of more than approximately 900° C., the pores become rearranged in the region of the silicon which will form the subsequent cavern at a porosity of approximately more than 70%, so that a single large pore, thus a cavity or a cavern is formed beneath the cover layer which has a low porosity. The pores on the top of the layer having a low porosity are mostly sealed in this high-temperature step, so that additional layers, e.g., one or more monocrystalline layers, may be deposited on the cover layer.

[0013] According to a preferred embodiment of the present invention, the etching medium and/or the etching media for creating the openings and/or pores in the cover layer and/or for creating the cavern is/are hydrofluoric acid (HF) or a liquid mixture or a chemical compound containing hydrogen fluoride.

[0014] In a preferred embodiment of the present invention, a highly volatile component, preferably an alcohol such as ethanol and/or purified water is added to the etching medium or etching media to dilute the etching medium or media.

[0015] Ethanol lowers the surface tension of an etching medium containing it, thus facilitating better wetting of the silicon surface and penetration of the etching medium into the etched pores or openings or cavities. Furthermore, the bubbles formed during the etching process are smaller than those formed without the addition of ethanol to the etching medium, and therefore the bubbles are better able to escape through the pores of the cover layer. Therefore, the pore size and/or porosity of the cover layer may be kept smaller in an advantageous manner than would be the case without the addition of the alcohol.

[0016] In another preferred embodiment of the present invention, the openings and/or pores in the cover layer and/or in the area of the subsequent cavern are to be produced by an electrochemical method, preferably using the etching medium or media mentioned above.

[0017] Furthermore, in a preferred embodiment of the present invention using an electrochemical etching process, preferably an etching process using hydrogen fluoride (HF), the rate of expansion of the pores or cavities formed in the etching process may be influenced by applying an electric voltage and an electric current induced thereby through the etching medium or media. The rate of expansion of the pores or cavities depends in particular on the doping of the silicon substrate to be etched, the current density, the HF concentration in the etching medium, as the case may be, and the temperature. It is self-evident that these are merely examples of relevant process parameters of an etching process according to the present invention.

[0018] According to a preferred embodiment of the present invention, the etching medium, the HF concentration in the etching medium, and/or the doping of the area to be etched and/or the temperature and, as the case may be, other process parameters of the etching process are selected so that the etching process, i.e., the formation of pores or cavities, may be adjusted in a suitable manner and/or stopped when the electric voltage is turned off, preferably as abruptly as possible.

[0019] In a preferred electrochemical etching process according to the present invention using a single etching medium and/or two or more etching media, in a first period of time during which the etching medium is in the area of the cover layer a first current density which is not necessarily constant over time is established in the etching medium. During a second period of time during which the particular etching medium is in the area of the cavern to be created, preferably a second current density, which is not necessarily constant over time but is higher or much higher than the or a current density established during the first period of time. In this way, the cavern or a precursor of the cavern is formed by pores or cavities, whose rate of expansion during the process of etching the cavern is higher or much higher than the rate of expansion of the pores for manufacturing the porous cover layer. In another preferred embodiment of the present invention, the area of the cover surface of the substrate or the epitaxial layer which is to be etched to form pores is to be surrounded by a masking layer, i.e., a supporting layer which allows free access of the etching medium or media to the area to be etched to form pores and which shields those areas of the cover layer of the substrate or the epitaxial layer which are not to be etched to form pores to prevent an etching attack there.

[0020] According to a preferred embodiment of the present invention, the supporting layer is designed so that it mechanically secures the region or the layer of the cover surface, which is to be etched to form pores, on the unetched portion of the substrate or the epitaxial layer, during and after the etching of the cavern.

[0021] In a preferred embodiment of the present invention, the supporting layer is created before etching the region or layer which is to be etched to form pores by first providing n-type doping on the nearest surrounding region around the layer of the cover surface to be etched to form pores on a p-doped silicon substrate. This makes it possible to largely prevent “underetching” of the substrate, in particular in the area in which the layer that is etched to form pores is mechanically joined to the silicon substrate. Otherwise there would be the risk, in particular in the case of a preferably thin porous layer, that the layer might separate from the substrate. In addition, a silicon nitride layer may be used as a mask and in particular to protect against an etching attack of electronic circuits which may be situated beneath it.

[0022] Alternatively or additionally, instead of the n-type doping and/or an n-doped layer, a metal layer or metal mask may be provided; this also largely prevents underetching of the substrate or the epitaxial layer. However, use of a metal layer or metal mask is usually expedient only if no circuits are to be provided in the substrate or in the epitaxial layer because otherwise metal atoms remaining in the substrate or in the epitaxial layer even after removal of the metal layer, i.e., the metal mask, could impair the functioning of the circuits.

[0023] In another preferred embodiment of the present invention, a porous etched cover layer, such as a silicon layer in particular, is to be pretreated before additional layers, preferably one or more silicon layers, are applied to, i.e., deposited on it. The goal of the pretreatment is to close the pores in the porous etched cover layer entirely or partially to further improve upon the quality of the silicon layer(s) applied thereto, i.e., deposited thereon, if necessary or expedient.

[0024] A pretreatment according to the present invention may include heating the porous etched cover layer, the heating being performed at a high temperature, e.g., at a temperature in the range from approximately 900° C. to approximately 1200° C. Heating is preferably performed under a hydrogen atmosphere, a nitrogen atmosphere and/or a noble gas atmosphere.

[0025] As an alternative or in addition to the aforementioned pretreatment, a (slight) oxidation of the porous etched silicon layer may be provided. The oxidation is preferably performed with (slight) addition of oxygen to the atmosphere to which the porous cover layer is exposed in the reactor, the oxidation preferably taking place at a temperature in the range from approximately 400° C. to 600° C. The term “slight” is understood to refer to oxidation which largely closes the pores of the porous etched cover layer entirely or partially and forms an approximately network-like oxide structure.

[0026] According to a preferred embodiment of the present invention, the slightly porous layer or cover layer is etched with an etching medium which has a hydrogen fluoride concentration (HF concentration) in the range from approximately 20% to approximately 50%, preferably approximately 30% to approximately 40%, in particular approximately 33%.

[0027] In another preferred embodiment of the present invention, the porous layer which forms a precursor of the subsequent cavity or cavern is etched with an etching medium which has a hydrofluoric acid concentration (HF concentration) in the range from approximately 0% to approximately 40%, preferably approximately 5% to approximately 20%, in particular less than approximately 20%. The remaining portion of the etching medium which is not formed by hydrofluoric acid is largely made up of an alcohol, such as ethanol in particular.

[0028] To achieve a high rate of expansion of the pores or cavities in the layer to be decomposed during an etching step, according to the present invention as defined above, to form a cavity or a cavern—a rate of expansion at which the pores or cavities “overlap” one another very rapidly and thus form a single “giant pore”—an etching medium according to the present invention is provided according to the present invention in an embodiment of the present invention. The etching medium according to the present invention has a hydrofluoric acid concentration (HF concentration) in the range from approximately 0% to approximately 5%, preferably approximately 1% to approximately 3%, in particular less than approximately 5%. The remaining portion of this etching medium not formed by hydrofluoric acid is composed mostly of an alcohol, such as ethanol in particular, and/or purified water.

DRAWING

[0029] The method according to the present invention for manufacturing a multilayer semiconductor component according to the present invention is explained in greater detail below using the example of heat transfer sensors using schematic drawings not necessarily drawn to scale, whereby the same reference numbers denote the same layers or parts or those having the same effect.

[0030]FIG. 1 shows a cross section of a precursor of a heat transfer sensor according to the present invention after producing a first silicon layer having a low porosity in a silicon substrate having a second porous silicon layer beneath the first silicon layer, the second layer having a higher porosity;

[0031]FIG. 2 shows a cross section of the precursor shown in FIG. 1 after the second silicon layer having a high porosity has become a cavity;

[0032]FIG. 3 shows a cross section of a precursor of a second variant of a heat transfer sensor having an epitaxial layer deposited on the silicon substrate;

[0033]FIG. 4 shows a cross section of a heat transfer sensor manufactured on the basis of the precursor shown in FIGS. 1 or 2 and provided with sensor elements;

[0034]FIG. 5 shows a cross section of a precursor of a third variant of a heat transfer sensor according to the present invention having a single thick porous layer;

[0035]FIG. 6 shows a cross section of a heat transfer sensor manufactured on the basis of the precursor depicted in FIG. 5 and provided with sensor elements; and

[0036]FIG. 7 shows a top view of a fourth variant of a heat transfer sensor according to the present invention having sensor elements which are provided for measuring the ambient pressure.

[0037]FIG. 1 shows a cross section of a precursor 100 of a heat transfer sensor 400 depicted in FIG. 4. To manufacture heat transfer sensor 400 depicted in FIG. 4, first a masking layer 102 is produced on the top of a silicon substrate 101, a region 103 not covered by masking layer 102 being formed. The masking layer may be, for example, a nitride layer, an n-doped layer (in the case of a p-doped silicon substrate), or some other suitable layer which is largely unattacked by the etching medium used subsequently. An integrated circuit 106 on the top of the silicon substrate is adequately protected from etching attack by masking layer 102.

[0038] The top of silicon substrate 101 is etched electrochemically using a suitable etching medium such that the etching medium creates small openings, i.e., pores in silicon substrate 101 directly beneath region 103 which is not covered. A silicon layer 104 having a low porosity is formed. The etching medium passes through these small openings, i.e., pores in silicon layer 104 to reach deep regions of silicon substrate 101 and also forms pores in the silicon there. A porous silicon layer 105 is formed here beneath porous silicon layer 104.

[0039] The etching medium for electrochemical etching, e.g., wet etching in particular, is preferably hydrofluoric acid (HF) or an etching medium which contains hydrofluoric acid (HF), among other things. According to the present invention, an electric field is preferably created between the top and the bottom of silicon substrate 101, the rate of expansion of the pores or openings or cavities being influenced via the electric field strength or the electric current density thus established.

[0040] In a preferred electrochemical etching method according to the present invention, precursors of the heat transfer sensors to be etched are placed in a pan-like container which is filled with the etching medium, and an electric voltage is applied at opposing ends of the etching medium so that the electric field is formed.

[0041] To ensure that porous silicon layer 104 is formed in the region directly beneath region 103 recessed beneath masking layer 102, an electric current density, which is not necessarily constant, is established in a first step, which is not covered after the etching medium is applied to region 103, which is not covered. The electric current density is preferably selected in such a way that openings or pores are formed in silicon substrate 101 directly beneath region 103 which is not covered.

[0042] Another important criterion for the electric current density, which is established in the first step and is not necessarily constant is to establish such an electric current density at which suitable openings or pores are formed in silicon substrate 101 directly beneath region 103 which is not covered. Suitable in particular are such openings or pores which subsequently allow the deposition of additional layers, preferably silicon layers, on porous silicon layer 104 formed during the etching process. Therefore, the openings or pores may have only an adequate size, i.e., an adequate diameter. Preferred openings or pores have a diameter of approximately 10 nm to 100 nm, for example, preferably approximately 10 nm to 30 nm.

[0043] It is self-evident that this is just one example of suitable openings or pores.

[0044] After the etching medium has penetrated through porous silicon layer 104, in a second step the current density is preferably increased in relation to the current density during the first step, so that the rate of expansion of the pores or cavities is increased and larger pores are allowed to form in silicon layer 105 in comparison with the pores in porous silicon layer 104.

[0045] The silicon decomposed by the etching medium is removed during the etching process and/or subsequently through the openings or pores in porous silicon layer 104 and “fresh” etching medium is supplied.

[0046] In the preferred first variant of the process according to the present invention for manufacturing the precursor of a heat transfer sensor, i.e., a cavity as shown in FIG. 1, the etching process for manufacturing subsequent cavity 201 (FIG. 2) is adjusted through the choice of suitable process parameters and/or one or more suitable etching media such that the porosity of silicon layer 105, forming subsequent cavity 201 is sufficiently high. The term “sufficiently” is preferably understood to refer to a porosity which is greater than 70% and less than 100%. A heating step is then performed. The heating is preferably performed under an atmosphere of hydrogen, nitrogen or noble gas and/or at a temperature of more than approximately 900° C. Because of the high porosity of silicon layer 105, the pores become rearranged during heating so that a single large pore is formed beneath slightly porous silicon layer 104, i.e., the cavity shown in FIG. 2 or cavern 201 depicted there. The pores on the top of silicon layer 104 having a low porosity are largely closed during heating, i.e., the high-temperature step, so that additional layers may be deposited on this layer.

[0047] In a second variant according to the present invention, which is not shown here but is also preferred, for manufacturing the precursor of a heat transfer sensor and/or a cavity 201, the process parameters are adjusted after forming low-porosity silicon layer 104 in such a way that the rate of expansion of the pores or cavities within a thin transitional layer beneath silicon layer 104 increases rapidly, and the pores in this transitional layer coalesce or more or less “overlap.” In other words, the transitional layer is a cavity which is initially superficial but grows in depth during the continued etching process, ultimately forming the cavity or cavern 201. In other words, not only are new pores created and then increased in size but also the transitional layer, an initially shallow superficial “giant pore,” grows slowly in depth.

[0048] According to the present invention, the etching medium and/or etching media are preferably provided with a readily volatile constituent; preferably an alcohol is used, e.g., ethanol.

[0049] If necessary or expedient, according to the present invention the porous region of the cover surface of substrate 101 which is to be etched to create pores is provided with a masking layer and/or a supporting layer, which secures the layer of the cover surface to be made porous by etching, i.e., silicon layer 104 mechanically (not shown) during and after the etching, i.e., during the creation of cavity 201 at the connecting points in the area of the unetched cover surface of the substrate. Such a supporting layer may be created, for example, by first providing the initially surrounding region around silicon layer 104, which is to be etched to make it porous, of the cover surface of p-doped silicon substrate 101 is provided with an n-type doping. This largely prevents “underetching” of silicon substrate 101 in the area of the connecting points, i.e., interfaces between silicon layer 104 and silicon substrate 101. Furthermore, care may be taken to ensure that even a preferably thin porous silicon layer 104 is reliably attached to silicon substrate 101.

[0050]FIG. 3 shows a precursor 300 of a second variant of a heat transfer sensor (not shown) in cross section. Precursor 300 shown here has a silicon substrate 101, a silicon epitaxial layer 301 deposited on silicon substrate 101 and a masking layer 102 applied to the top of silicon epitaxial layer 301. Masking layer 102 has a region 103 which does not cover silicon epitaxial layer 301. Furthermore, an integrated circuit 303 and/or 302 is formed in the top of silicon epitaxial layer 301 and between the silicon substrate, i.e., wafer 101 and epitaxial layer 301.

[0051] Using the method described here, a porous silicon layer 104 is created in silicon epitaxial layer 301 and a cavity or a cavern 201 is created in region 103 beneath that.

[0052] Both variants of the heat transfer sensors depicted in FIGS. 1 through 3 have a porous load-bearing cover layer 104 above a cavity or a cavern 201 so that additional layers may be deposited on the cover layer. The thickness of porous cover layer 104 amounts to approximately 2 μm to 10 μm, preferably approximately 3 μm to 5 μm. In addition, one or more defined openings may be introduced into cover layer 104 to achieve pressure equalization between cavity 201 and the environment.

[0053]FIG. 4 shows a heat transfer sensor 400 manufactured on the basis of the precursor depicted in FIGS. 1 or 2, shown in cross section. After removing masking layer 102, porous silicon layer 104 is sealed by deposition of a sealing layer 401 or by oxidation. The pressure prevailing in deposition of sealing layer 401 or in oxidation defines the pressure enclosed in the cavity or in cavern 201. The thickness and porosity of the silicon layer are preferably such that the layer has an adequate mechanical stability with respect to the substrate under the boundary conditions of the lowest possible thermal conductivity, i.e., the best possible thermal insulation of the sensor region with respect to the substrate. Preferably a pressure which is reduced with respect to atmospheric pressure, particularly a vacuum is enclosed in the cavity or in cavern 201 to achieve a better thermal insulation between a sensor region 404 provided above porous silicon layer 104 and silicon substrate 101. This further reduces the transfer of heat from the sensor area to silicon substrate 101 and thus yields a higher response sensitivity, i.e., response rate of sensor elements situated in the sensor region.

[0054] If after deposition of sealing layer 401, gas, e.g., there is hydrogen in the cavity or in cavern 201, it may be mostly removed from the cavity or cavern 201, using a high-temperature step according to the present invention, e.g., under a nitrogen atmosphere. Because of its small molecule size and the hydrogen concentration gradient, hydrogen diffuses in particular through sealing layer 401, which is usually thinner in relation to the substrate.

[0055] The gas-tight, i.e., airtight sealing of the cavity or cavern 201 may be performed in a reactor or in a vapor deposition system and need not necessarily be performed after the manufacturing the cavity or cavern 201 but instead may be performed as one of the last method steps. This has the advantage that no warps develop in porous layer 104 which might result in imaging errors in a subsequent optical structuring operation (lithography).

[0056] Before deposition of sealing layer 401, porous silicon layer 104 may be pretreated. A preferred pretreatment according to the present invention involves heating porous silicon layer 104. Heating is preferably performed at a high temperature, e.g., at a temperature in the range from approximately 900° C. to approximately 1200° C. and/or heating is performed under an atmosphere of hydrogen, nitrogen, and/or noble gas.

[0057] The pretreatment makes it possible to mostly seal the pores in porous etched monocrystalline silicon layer 104 so that additional layers, silicon layers in particular, may be deposited on it. It is self-evident that such a pretreatment may be omitted, in particular for reasons of cost, if the quality of the deposited silicon layers is satisfactory even without a pretreatment.

[0058] Sealing layer 401 may be formed from one or more layers of silicon oxide, silicon nitride, polysilicon, or monocrystalline silicon. Due to the deposition of different silicon layers, e.g., silicon oxide and silicon nitride layers, a tensile stress is produced in sealing layer 401, counteracting the pressure difference between the top of the membrane and the bottom of the membrane so that warping in sealing layer 401 which may be caused due to the pressure difference is prevented.

[0059] In sensor area 404, sensor elements 402, 403 are created above porous silicon layer 104 on sealing layer 401, i.e., on oxidized silicon layer 104; such sensor elements are formed, for example, by deposition and structuring of metal, preferably platinum. As an alternative, sensor elements 402 and 403 may be created by deposition of polysilicon on sealing layer 401, for example, subsequent doping of the deposited polysilicon and then structuring the deposited polysilicon layer. Furthermore, sensor elements 402 and 403 may be created by deposition of a polysilicon layer and structured doping of the polysilicon layer (not shown). Sensor elements 402 and 403 include resistors which function as temperature sensors and as heating elements, temperature sensor 402 being situated in outer sensor area 404 and heating elements 403 being positioned in the internal sensor area. Preferably only one heating element 403 is operated, and the second heating element 403 functions as a reserve when the first heating element fails. A stream of air or gas flowing in direction of flow S over the surface of heat transfer sensor 400 receives the heat released by heating elements 403, resulting in a temperature gradient between two temperature sensors 402. Heat transfer sensor 400 may be situated, for example, in the intake channel of a motor vehicle for measuring and/or controlling the combustion air supplied to it. On the basis of the temperature gradient, which is analyzed by integrated circuit 106, for example, it is possible to determine the quantity of air or gas flowing over heat transfer sensor 400 and its direction of flow.

[0060] Additional layers may be applied to sensor elements 402 and 403 and structured if necessary. For example, one or more silicon nitride layers may be deposited to protect sensor elements 402 and 403 from penetration of moisture and/or to increase the mechanical stability of sensor elements 402 and 403. The thickness of the layer(s) deposited on sensor elements 402 and 403 is preferably such that an adequate response sensitivity of sensor elements 402 and 403 remains ensured, i.e., the thermal capacity of the layer or layers should be as low as possible.

[0061]FIG. 5 shows a cross section of a precursor 500 of a second variant of a heat transfer sensor 600 according to the present invention (FIG. 6) having a single thick porous layer 501. The thickness of porous layer 501 is created by a method similar to that described in particular in conjunction with FIGS. 1 through 3. However, porous layer 501 is preferably much thicker than layer 104 which has a low porosity, the thickness of porous layer 501 preferably being approximately 40 μm to 80 μm, in particular approximately 50 μm to 60 μm. In contrast with the heat transfer sensors described above, there is no provision here for formation of a cavity or a cavern 201. The porosity and/or pore size of porous layer 501 may be optimized by the etching method described above to greatly reduce the thermal conductivity. For example, the porosity and/or pore size may be varied within-porous layer 501. The porosity and/or pore size is preferably larger in the lower region of porous layer 501 which faces substrate 101 than in the upper region of porous layer 501, which faces sensor region 404. For example, the porosity in the upper region of porous layer 501 is up to max. 20% and in the lower region of porous layer 501 is up to max. 80%. Additionally or alternatively, thick porous layer 501 may be oxidized to further reduce thermal conductivity. A single thick porous layer 501 has the advantage in particular of a greater mechanical stability in comparison with a combination of a thin porous layer 104 and a cavern 201 and usually has the disadvantage of higher thermal conductivity, i.e., greater transfer of heat from the sensor region to the substrate.

[0062]FIG. 6 shows a cross section through a heat transfer sensor 600 manufactured on the basis of the precursor depicted in FIG. 5. After removal of masking layer 102, one or more layers may be deposited on thick porous layer 501 and structured if necessary, with an airtight sealing of porous layer 501 optionally being provided. A sealing layer 401 is preferably deposited on thick porous layer 501 and includes, according to the exemplary embodiment in FIG. 4, one or more silicon layers, e.g., silicon oxide, silicon nitride, polysilicon or monocrystalline silicon layers. The sensor elements, i.e., resistors 402 and 403, are created on sealing layer 401 by deposition and structuring of metal, preferably platinum. Additional layers may be deposited on sensor elements 402 and 403 and structured.

[0063] Heat transfer sensor 600 is designed similarly to heat transfer sensor 400 in FIG. 4 for measuring air and/or gas masses so that the functioning of heat transfer sensor 600 is identical to the functioning of heat transfer sensor 400 according to FIG. 4. Heat transfer sensors 400 and 600 are preferably used in the automotive area, in particular in the intake area of motor vehicles to detect the intake currents for the purpose of optimizing the combustion mixture. However, all other areas of application in which air or gas masses are to be determined are also conceivable.

[0064] Heat transfer sensor 400 according to FIG. 4 has a very good thermal insulation of the sensor region with respect to the silicon substrate because of porous layer 104 and because of the formation of the cavity or cavern 201 beneath porous layer 104, whereas heat transfer sensor 600 according to FIG. 6 has a great mechanical stability with an increased manufacturing yield. In particular when the heat transfer sensor is positioned in the intake channel, it is exposed to strong air currents or gas currents and to bombardment with particles, so an adequate mechanical stability must be ensured to achieve the longest possible lifetime. Furthermore, adequate thermal insulation, which ensures a satisfactory response sensitivity, is obtained due to the formation of a thick porous layer 501 having sufficient mechanical stability, preferably with a varying porosity and/or pore size.

[0065]FIG. 7 shows a top view of a fourth variant of a heat transfer sensor 700, its sensor elements 703 and 704 being provided for measurement of the pressure of a gas, e.g., hydrogen surrounding the sensor. Using the method according to the present invention as described above, a porous silicon layer (not shown) is created in silicon substrate 101 with a cavity or a cavern 701 beneath it, as depicted in FIGS. 1 and 2 for precursors 100 and 200 of heat transfer sensor 400. A sealing layer 401 is deposited on the porous layer and then sensor elements 703 and 704 are created on this layer according to the method explained with reference to FIG. 4. In addition, one or more integrated circuits may be provided for analysis of the measurement signals. Sensor elements 703 and 704 are designed as resistors, sensor element 703 functioning as a temperature sensor and sensor element 704 functioning as a heating element. In addition, one or more heating elements (not shown) may also be provided to be able to switch from one heating element to the other in the event of damage to a heating element. Heating element 704 and temperature sensor 703 are situated next to one another and are connected to contact surfaces 706 via leads 705.

[0066] In operation, heating element 704 is heated by electric current and temperature sensor 703 measures the temperature. The heat emitted by the heating element to the gas surrounding the sensor depends on its pressure and its type, i.e., the greater the ambient pressure, the greater is the quantity of heat transferred from the heating element to the gas. The ambient pressure and/or the type, i.e., composition of the gas may be determined on the basis of the heat output which is necessary to maintain a defined, i.e., constant, temperature.

[0067] To prevent dissipation of heat from heating element 704 into sealing layer 401 and into the porous layer, sealing layer 401 and porous layer 104 have been partially removed at the sides of sensor elements 703 and 704. The porous layer and sealing layer 401 form a web 702 above the cavity or cavern 701 which is open on one side. As an alternative, sealing layer 401 alone may be removed from the sides of sensor elements 703 and 704.

[0068] As an alternative, formation of a cavity or cavern may be omitted according to the exemplary embodiment as depicted in FIGS. 5 and 6, and instead a single thick porous layer (not shown) which is preferably oxidized may be provided. A sealing layer 401 which completely covers the porous layer or covers only a portion, e.g., as a web 702, is applied to the thick porous layer. As described above, sensor elements 703 and 704 are produced on sealing layer 401.

[0069] If necessary, additional layers may be deposited on sensor elements 703 and 704 and optionally structured. 

What is claimed is:
 1. A method for manufacturing a semiconductor component (100; . . . ; 700), a multilayer semiconductor component in particular, preferably a micromechanical component, such as, in particular, a heat transfer sensor, the semiconductor component having a semiconductor substrate (101), in particular made of silicon; and a sensor region (404); where in a first step, a porous layer (104; 501) is formed, which thermally insulates the sensor region (404) at least partially from the semiconductor substrate (101); and in a second step, a cavity or a cavern (201; 701) is formed beneath the first porous layer (104) in the semiconductor component; and the second step has a first substep, during which an initially superficial cavity is formed beneath the first porous layer (104), and the initially superficial cavity is expanded in depth, thereby forming the cavity or cavern (201; 701) from the initially superficial cavity.
 2. A method for manufacturing a semiconductor component (100; . . . ; 700), a multilayer semiconductor component in particular, preferably a micromechanical component such as, in particular, a heat transfer sensor, the semiconductor component having a semiconductor substrate (101), in particular made of silicon; and a sensor region (404); where in a first step, a porous layer (104; 501) is formed, which thermally insulates the sensor region (404) at least partially from the semiconductor substrate (101); and in a second step, a cavity or a cavern (201; 701) is formed beneath the first porous layer (104) in the semiconductor component; and the second step has a first substep, during which a second porous layer (105) having a porosity of more than approx. 70% and less than 100%, preferably approx. 80%, is formed beneath the first porous layer (104); and the cavity or the cavern (201; 701) is formed from the second porous layer (105) by a heating step.
 3. The method as recited in one of claims 1 or 2, wherein the first and/or second porous layers (104, 105) are formed by one or more etching media, the etching medium and/or the etching media preferably having hydrofluoric acid or being made of hydrofluoric acid.
 4. The method as recited in claim 3, wherein the etching medium or etching media is/are provided with one or more additives, such as additives to reduce the formation of bubbles, to improve wetting and/or to improve drying, such as an alcohol in particular, e.g. ethanol, the volume concentration of the additive, e.g. ethanol in particular, preferably amounting to approx. 30% to approx. 80% in the case of ethanol.
 5. The method as recited in one of claims 1 through 4, wherein the first and/or second porous layers (104, 105) are formed by applying an electric field between the top and the bottom of the semiconductor component (100; . . . ; 700) and establishing an electric current.
 6. The method as recited in one of claims 1 through 5, wherein the method parameters for formation of the second porous layer (105) and/or for formation of the initially superficial cavity are selected in such a way, that the rate of expansion of the pores or cavities in the second porous layer is much higher than the rate of expansion of the pores or cavities to form the first porous layer (104).
 7. The method as recited in one of claims 1 or 3 through 6, wherein the method parameters for formation of the initially superficial cavity are selected in such a way, that the pores or cavities of the second porous layer (105) “overlap” with one another in the lateral direction, and thus, a single, initially superficial pore or a single, initially superficial cavity, is formed.
 8. The method as recited in one of claims 3 through 7, wherein the doping of the semiconductor substrate (101) to be etched, a silicon substrate in particular, the current density in the etching medium or etching media, the hydrofluoric acid concentration in the etching medium or etching media, one or more additives to the etching medium or etching media, and the temperature, represent method parameters.
 9. The method as recited in one of claims 1 through 8, wherein hydrogen enclosed in the cavern or the cavity (201) is substantially, completely or partially removed from the cavern or cavity within the scope of a high-temperature step.
 10. A semiconductor component (100; 200; 300; 400; 700), wherein it is manufactured by a method as recited in one or more of claims 1 through
 9. 11. The semiconductor component (100; . . . ; 700), in particular as recited in claim 10, a multilayer semiconductor component in particular, preferably a micromechanical component such as, in particular, a heat transfer sensor, being provided as a semiconductor component, the semiconductor component having a semiconductor substrate (101), in particular made out of silicon, and a sensor region (404) and a porous layer (104; 501), which thermally insulates the sensor region (404) at least partially from the semiconductor substrate (101), where a sealing layer (401) is applied to the porous layer (104; 501), the sealing layer (401) preferably including one or more silicon layers, e.g., silicon oxide, silicon nitride and/or similar silicon layers, and the layer sequence, i.e. the layer structure of the sealing layer (401) is selected, i.e. designed, in such a way that a tensile stress is formed in the sealing layer (401) to at least partially compensate for a pressure gradient between the top and the bottom of the sealing layer. 